In electronics, it is often useful to measure and sense currents and voltages present in a circuit. Voltage measurements are made by connecting a positive and negative lead of the input of a differential amplifier to two locations in a circuit. When the negative lead is ground or reference ground, it is assumed to have a voltage of zero. In this case, the voltage measured is called a single-ended measurement of the voltage at the positive lead. When the negative lead is one side and the positive lead is another, the voltage measured is the difference between the voltage at the positive side and negative side. This voltage measurement is called a differential voltage measurement.
Generally current measurements are more difficult than voltage measurements. One approach is to insert a known, generally small resistance in series with the current and measure the voltage drop across the resistance. Another approach is to measure the integrated magnetic field around the conductor. The former approach involves modification of the circuit and causes losses. The latter approach also may involve modification of the circuit in some manner to expose a path to loop around the conductor. Sometimes, there are other ways to infer current from a particular voltage measurement.
In test and measurement applications, there is often a need to probe currents and voltages. Probing may involve measuring currents and voltages, often for design, analysis, validation, and/or debugging of various electronic circuits. Voltage probes for test and measurement are most common, and single-ended voltage probes are the most common type of voltage probe. Differential voltage probes are more difficult and often need to deal not only with measuring differential voltages, but with rejecting unwanted common-mode voltages. Current probes are usually available in traditional forms for low frequency applications and include a wire or conductor carrying the current capable of looping the probes around. The wire or conductor of the probe is often included as part of a design constraint requirement an element of the probe to surround the current carrying conductor. This constraint may be difficult to work around when probing existing circuits.
In test and measurement situations, needs arise involving probing of voltages and currents in switch-mode power supplies (SMPS). The design and analysis of power delivery systems such as SMPS fall under a broad category called power integrity (PI). These SMPS are critical to most of today's electronic circuits and often require extensive analysis of output impedance, stability, and other behavior and parameters.
FIG. 2 depicts a basic schematic diagram of a SMPS. The SMPS comprises voltage regulation module (VRM) 4, which controls output voltage 12 under changing load currents through load impedance 13 and changing input voltage 8. In many applications, VRM 4 may regulate output voltage 12 by modulating switch node voltage 11. For example, VRM 4 may change the duty cycle of two switches: high-side field-effect transistor (FET) 5 that connects one side of inductor 7 to input voltage 8 and low-side FET 6 that connects the same side of inductor 7 to a low voltage (usually ground). The other side of inductor 7 connects to output voltage 12 that remains essentially constant, delivering output current 15 to load impedance 13 reference to a device under test (DUT) reference ground 16. A feedback network 14 feeds back measurements of output voltage 12 and/or sometimes measurements of output current 15.
In many high-current, high-power applications, it is common for multiple phases to supply current to the output. In multi-phase systems, the current is supplied through multiple inductors that are independently switched in a coordinated effort by a VRM. In the design, analysis, validation and debugging of multi-phase systems, it is often necessary to measure the current sharing between the multiple phases. In these applications in particular, it is useful to measure the currents through the multiple inductors, particularly under transient output load currents. Unfortunately, there is usually no opportunity to break the circuit for connection of a traditional current measurement probe.
Current can be calculated directly from the voltage drop across an inductor with an internal parasitic resistance, but the dynamic range required to make accurate measurements is beyond the capabilities of most measurement instruments. To address this, a method was determined for inductor current measurement and published as Linfinity, “A simple current-sense technique eliminating a sense resistor.”, AN-7, July 1998, which provides a method used by SMPS and VRM designers to sense inductor current particularly for over-current detection and crowbar circuitry to shut down the system in the event of various fault conditions. In this application note, the authors point out the need for matching the time-constant formed by the inductor and internal parasitic resistance and an RC network formed by shunting the inductor with a series resistor-capacitor combination. In the intended use of this application, precise matching is not required because precise equalization of the low-frequency current measurement and higher frequency switching current is not necessary as it is used primarily for fault current detection. Furthermore, in the intended application, the RC network would be designed into the circuit and would not need to be added later. Using this application note, some engineers utilizing SMPS in their systems, hand-solder RC networks into their systems to make these measurements in test and measurement applications. The use of the methods put forth in this application note requires careful handling of the circuit and calculation of matching values. Without precise matching of circuit element values, this application note is not well suited for test and measurement applications and needs additional improvement.
In U.S. Pat. No. 8,289,037, filed Sep. 30, 2009, titled “Method and Apparatus to Measure Current in Power Switchers” to Labib et al., various methods for inductor current measurements and their drawbacks are surveyed, preferring the method provided in Linfinity AN-7 with the addition of methods of providing for determination of the inductor parasitic resistance through calibration circuitry. This inductor parasitic resistance sets the low-frequency portion gain of the system in converting the measured voltage to inductor current, but does not enable calibration of high-frequency portion or the general frequency response of the measurement system. Labib et al. is silent regarding calibration of the full frequency response, and as it provides no mechanism or method for adjusting the RC network resistor and/or the RC network capacitor, nor any method for processing the acquired differential voltage, it is inadequate for complete, precise, and accurate inductor current measurement.
The approaches described in this section are approaches that could be pursued, but not necessarily approaches that have been previously conceived or pursued. Therefore, unless otherwise indicated, it should not be assumed that any of the approaches described in this section qualify as prior art merely by virtue of their inclusion in this section.